Plate/screen voltage sequencing and cathode bias

[KK7UV]

I'm making great progress on the 813 transmitter build.   Some questions for now...

1.  I have separate plate and screen supplies.  I know the screen voltage must not come on before the plate voltage, but is it okay to have them come on/off at the same time in the same sequencer step?

2.  The cathodes of the PA and modulator are biased during standby with 50k resistors, shorted by a relay during transmit.  At what point in the T-R sequence should this occur?   What are the standby cathode voltages when lifted by 50kΩ?  And are those bias resistors needed at all if the plate and screen are off during RX/standby? 

Thanks,
Steve KK7UV

[K1JJ]

Hi Steve,

Good to hear about your 813 progress... and employing the GFZ audio driver board.  It sounds like a FB rig.


Take a look at pages 4 to 9  in this 4-1000A plate modulated "Fabio" thread.  We all beat the sequencing subject to death.  You should find many ideas to proceed.   If you still have questions, ask away.

http://amfone.net/Amforum/index.php?topic=33816.75


Fabio is still working FB with this elaborate  key up / unkey sequencing box.   The key is being able to adjust key up and unkey timing and sequencing order if you run into problems.

BTW, the one thing that helped to cure my T/R arcing problems was putting a 5K power resistor in series with a vac relay across the mod  reactor that quenched the arc on unkey.   The reactor/mod iron power has to go somewhere.   Start with that technique and go from there. It may simplify things for you.

** The cathode CT voltages always remain at low voltage with the 50K (cutoff bias) across the contacts to ground.   A small 3A relay with a 50K  two watt resistor will work FB there.   Just like the commercial linear amps.

T


*** Coming soon:  "The Summer of Antennas."     Watch for a new thread featuring ten new multi-element antenna arrays -  designed, built and raised at K1JJ over the summer/fall.  Final testing and optimization is in progress.   Pictures and performance data coming soon....

[w9jsw]

On my 813 rig I have 2 sequences. for TX, Group 1 comes up, then group 2. to go back to RX, group 2 drops then group 1. I also use the K1JJ snuffer across the mod trans secondary (the side that attaches to the mod reactor)


I shut off HV during RX

Group 1 - antenna T/R relays

Group 2 - modulator CT, Plate CT, screen, HV, drive, snuffer

Here is the sequencer.

I get my drive by telling the SDR to go into CW mode with the CW key jack. The sequencer simply closes a relay to make that happen. My SDR will emit a CW tone even if it is in a different mode for RX. Sweet.

[KK7UV]

Tom - That was a good read last night regarding your tribulations with Fabio 2 sequencing.  Thanks!  that gave me much to consider in how to troubleshoot any sequencing problem I might have when I get to that stage of testing.

I will have snubbers across both mod. and screen reactors - controlled by vac. relays.

John:  thanks for your sequencer diagram.   so your mod. and p.a. center-taps have the 50k resistors?

[w9jsw]

yes, per the k1jj maul design.